A formation tester tool is deployed downhole in a well bore. The formation tester (FT) withdraws fluids out of the formation by pumping. Initial pressure versus rate response is used to estimate basic reservoir parameters such as fluid mobility and reservoir pressure. Subsequent pumping is then used to acquire representative formation fluid samples. Because of the presence of mud filtrate, it takes some time to get pure formation fluids (fluid from oil zone). Therefore, OCM is applied to monitor the degree of contamination and determine when uncontaminated formation fluids are being sampled.
Oil based mud (OBM) contamination poses a major challenge to the acquisition of representative formation fluids using wireline formation testers (FT). There is a need for a real time determination of OBM to allow for a representative sample to be extracted from a downhole wellbore.
However, predicted OBM contamination from OCM is not particularly accurate and is variable. The performance of OCM changes in real time as a function of fluid volume withdrawn. Moreover, because the measurements are optical-density based, the relative error in OCM predictions increases with decreasing contamination levels and/or with smaller color/density/gas-oil ratio (GOR) contrast between reservoir fluids and contamination. It is in these cases that increased accuracy is needed to ensure the collection of representative formation fluid samples.
The present invention provides real time oil-based-mud contamination monitoring during formation testing having increased accuracy and precision. This allows for sampling of fluids in the wellbore that are not contaminated.